Zinc oxide

ABSTRACT

A dispersion containing zinc oxide particles having a dispersion particle size of (i) median volume particle diameter in the range from 70 to 130 nm, (ii) less than 16% by volume of particles having a volume diameter of less than 35 nm below the median volume particle diameter, and (iii) more than 84% by volume of particles having a volume diameter of less than 57 nm above the median volume particle diameter. The zinc oxide dispersion can be used in a sunscreen product that exhibits both effective UV protection and improved transparency. The zinc oxide is particularly suitable for use in combination with transparent titanium dioxide.

FIELD OF INVENTION

The present invention relates to zinc oxide, in particular in the formof a dispersion, and to the use thereof in a sunscreen product.

BACKGROUND

Zinc oxide has been employed as an attenuator of ultraviolet light incosmetic and sunscreen applications. Due to the increased awareness ofthe link between ultraviolet light and skin cancer, there has been anincreasing requirement for ultraviolet light protection in everydayskincare and cosmetics products. Unfortunately, existing commerciallyavailable zinc oxide products are not sufficiently transparent and canhave an unacceptable whitening effect when used on the skin. There is aneed for a zinc oxide in a form, particularly when used in combinationwith titanium dioxide, which exhibits improved transparency, reducedwhitening, and provides broad spectrum ultraviolet light protection.

REVIEW OF THE PRIOR ART

EP-0535971-B is directed to an oil dispersion of zinc oxide particles.

SUMMARY OF THE INVENTION

We have now surprisingly discovered an improved zinc oxide, whichovercomes or significantly reduces at least one of the aforementionedproblems.

Accordingly, the present invention provides a dispersion comprisingparticles of zinc oxide in a dispersing medium wherein the zinc oxideparticles in dispersion have (i) a median volume particle diameter inthe range from 70 to 130 nm, (ii) less than 16% by volume of particleshaving a volume diameter of less than 35 nm below the median volumeparticle diameter, and (iii) more than 84% by volume of particles havinga volume diameter of less than 57 nm above the median volume particlediameter.

The invention also provides a particulate zinc oxide having a dispersionparticle size of (i) median volume particle diameter in the range from70 to 130 nm, (ii) less than 16% by volume of particles having a volumediameter of less than 35 nm below the median volume particle diameter,and (iii) more than 84% by volume of particles having a volume diameterof less than 57 nm above the median volume particle diameter.

The invention further provides a particulate zinc oxide having anextinction coefficient at 524 nm (E₅₂₄) in the range from 0.1 to 1.0l/g/cm an extinction coefficient at 450 nm (E₄₅₀) in the range from 0.3to 2 l/g/cm an extinction coefficient at 360 nm (E₃₆₀) in the range from11 to 20 l/g/cm an extinction coefficient at 308 nm (E₃₀₈) in the rangefrom 11 to 20 l/g/cm a maximum extinction coefficient E(max) in therange from 12 to 20 l/g/cm, and a λ(max) in the range from 363 to 377nm.

The invention further provides a sunscreen product comprising a zincoxide or dispersion as defined herein.

The invention further provides the use of a zinc oxide or dispersion asdefined herein in the manufacture of a sunscreen having reducedwhiteness.

The invention still further provides a sunscreen product comprising (a)zinc oxide having a dispersion particle size of (i) median volumeparticle diameter in the range from 70 to 130 nm, (ii) less than 16% byvolume of particles having a volume diameter of less than 35 nm belowthe median volume particle diameter, and (iii) more than 84% by volumeof particles having a volume diameter of less than 57 nm above themedian volume particle diameter, and (b) titanium dioxide having adispersion particle size of (i) median volume particle diameter in therange from 24 to 42 nm, (ii) less than 16% by volume of particles havinga volume diameter of less than 15 nm below the median volume particlediameter, and (iii) more than 84% by volume of particles having a volumediameter of less than 20 nm above the median volume particle diameter.

The particulate zinc oxide used in the present invention comprisesprimary particles suitably having a mean particle size (measured asdescribed herein) in the range from 35 to 65 nm, preferably 40 to 60 nm,more preferably 45 to 55 nm, particularly 48 to 52 nm, and especially 49to 51 nm. The size distribution of the primary zinc oxide particles canhave a significant effect on the final properties of, for example, asunscreen product comprising the zinc oxide. In a preferred embodimentof the invention suitably at least 50%, preferably at least 60%, morepreferably at least 70%, particularly at least 80%, and especially atleast 90% by number of particles have a particle size within the abovepreferred ranges given for the mean particle size.

The primary zinc oxide particles are preferably approximately spherical,preferably having a mean aspect ratio d₁:d₂ (where d₁ and d₂,respectively, are the length and width of the particle (measured asdescribed herein)) in the range from 0.6 to 1.4:1, more preferably 0.7to 1.3:1, particularly 0.8 to 1.2:1, and especially 0.9 to 1.1:1. In apreferred embodiment of the invention, suitably at least 40%, preferablyat least 55%, more preferably at least 70%, particularly at least 80%,and especially at least 90% by number of particles have an aspect ratiowithin the above preferred ranges given for the mean aspect ratio.

When formed into a dispersion according to the present invention, theparticulate zinc oxide suitably has a median volume particle diameter(equivalent spherical diameter corresponding to 50% of the volume of allthe particles, read on the cumulative distribution curve relating volume% to the diameter of the particles—often referred to as the “D(v,0.5)”value)) (hereinafter referred to as dispersion particle size), measuredas herein described, in the range from 70 to 130 nm, suitably 80 to 120nm, preferably 87 to 113, more preferably 93 to 107 nm, particularly 97to 103 nm, and especially 99 to 101 nm.

The size distribution of the zinc oxide particles in dispersion can alsobe an important parameter in obtaining, for example, a sunscreen producthaving the required properties. The zinc oxide particles suitably haveless than 16% by volume of particles having a volume diameter of lessthan 35 nm, preferably less than 33 nm, more preferably less than 30 nm,particularly less than 25 nm, and especially less than 20 nm below themedian volume particle diameter. In addition, the zinc oxide particlessuitably have less than 30% by volume of particles having a volumediameter of less than 19 nm, preferably less than 18 nm, more preferablyless than 16 nm, particularly less than 12 nm, and especially less than8 nm below the median volume particle diameter.

Further, the zinc oxide particles in dispersion suitably have more than84% by volume of particles having a volume diameter of less than 57 nm,preferably less than 54 nm, more preferably less than 50 nm,particularly less than 45 nm, and especially less than 40 nm above themedian volume particle diameter. Also, the zinc oxide particles suitablyhave more than 70% by volume of particles having a volume diameter ofless than 24 nm, preferably less than 23 nm, more preferably less than21 nm, particularly less than 16 nm, and especially less than 10 nmabove the median volume particle diameter.

It is preferred that none of the zinc oxide particles in dispersionshould have an actual particle size exceeding 200 nm. Particlesexceeding such a size may be removed by milling processes which areknown in the art. However, milling operations are not always totallysuccessful in eliminating all particles greater than a chosen size. Inpractice, therefore, the size of 95%, preferably 99% by volume of theparticles should not exceed 200 nm, preferably 150 nm.

Particle size of the zinc oxide particles in dispersion may be measuredby electron microscope, coulter counter, sedimentation analysis andstatic or dynamic light scattering. Techniques based on sedimentationanalysis are preferred. The median particle size may be determined byplotting a cumulative distribution curve representing the percentage ofparticle volume below chosen particle sizes and measuring the 50thpercentile. The median particle volume diameter of the zinc oxideparticles in dispersion is suitably measured using a Brookhaven particlesizer, as described herein. The particle size distributions can also beobtained from the same cumulative distribution curve.

In a particularly preferred embodiment of the invention, the zinc oxideparticles suitably have a BET specific surface area (measured asdescribed herein) in the range from 10 to 40, preferably 15 to 35, morepreferably 20 to 30, particularly 23 to 27, and especially 24 to 26m²/g.

The particulate zinc oxide used in the present invention may be formedby any suitable process and typical processes are the French Method inwhich metallic zinc is melted and evaporated before being oxidized inthe gas phase, the American method in which zinc ores are sintered andreduced with cokes and the zinc thus obtained is oxidised to zinc oxide,and a wet method in which a water soluble zinc salt such as zincchloride or zinc sulphate is crystallised and then converted to zincoxide by sintering. Fractionation techniques, as known in the art, e.g.micronisation, sedimentation, or centrifugation, may be employed inorder to obtain zinc oxide having the required particle size and sizedistribution as defined herein.

The particles of zinc oxide may comprise substantially pure zinc oxide,but in one embodiment of the invention the particles have an inorganicand/or organic coating. The inorganic coating is preferably one or moreoxides or hydrous oxides, for example aluminium, silicon, titanium,zirconium, magnesium or zinc. The organic coating may be a fatty acid,an organic silicon compound, polyol, amine and/or alkanolamine. Thecoating is usually chosen to ensure compatibility with the particularmedium that will be used with the zinc oxide particles. Thus, inorganichydrophilic coatings are normally preferred for incorporating the zincoxide particles in aqueous media, and organic hydrophobic coatings fororganic, particularly oil, media.

The level of purity of the zinc oxide particles is an importantrequirement for use, for example, in cosmetic and sunscreenapplications. In a preferred embodiment, the lead content of the zincoxide particles (uncoated and/or coated) is preferably less than 15 ppm,more preferably less than 13 ppm, particularly less than 10 ppm, andespecially less than 6 ppm.

The zinc oxide particles used in the present invention exhibit improvedtransparency suitably having an extinction coefficient at 524 nm (E₅₂₄)(measured as described herein) of less than 1.5, preferably less than1.2, more preferably in the range from 0.1 to 1.0, particularly 0.3 to0.9, and especially 0.5 to 0.8 l/g/cm. In addition, the zinc oxideparticles suitably have an extinction coefficient at 450 nm (E₄₅₀)(measured as described herein) of less than 3, preferably less than 2.5,more preferably in the range from 0.3 to 2, particularly 0.6 to 1.7, andespecially 1 to 1.5 l/g/cm.

The zinc oxide particles exhibit effective UV absorption, suitablyhaving an extinction coefficient at 360 nm (E₃₆₀) (measured as describedherein) of greater than 10, preferably in the range from 11 to 20, morepreferably 12 to 17, particularly 13 to 15, and especially 13.5 to 14.5l/g/cm. The zinc oxide particles also suitably have an extinctioncoefficient at 308 nm (E₃₀₈) (measured as described herein) of greaterthan 10, preferably in the range from 11 to 20, more preferably 11.5 to16, particularly 12 to 14, and especially 12.5 to 13.5 l/g/cm.

The zinc oxide particles suitably have a maximum extinction coefficientE(max) (measured as described herein) in the range from 10 to 25,preferably 12 to 20, more preferably 13 to 18, particularly 14 to 17,and especially 15 to 16 l/g/cm. The zinc oxide particles suitably have aλ(max) (measured as described herein) in the range from 360 to 380,preferably 363 to 377, more preferably 366 to 375, particularly 368 to373, and especially 369 to 372 nm.

The zinc oxide particles can exhibit reduced whiteness, suitably havinga change in whiteness ΔL of a sunscreen product containing the particles(measured as described herein) of less than 3.5, preferably less than 3,more preferably less than 2.5, particularly less than 2.0, andespecially less than 1.5. In addition, a sunscreen product containingthe zinc oxide particles suitably has a whiteness index (measured asdescribed herein) of less than 90%, preferably in the range from 5 to80%, more preferably 10 to 70%, particularly 15 to 60%, and especially20 to 50%.

A composition, preferably a sunscreen product, containing the zinc oxideparticles defined herein as substantially the sole sunscreen agent,suitably has a Sun Protection Factor (SPF) (measured as describedherein) of greater than 4, preferably greater than 6, more preferablygreater than 9, particularly in the range from 12 to 20, and especially15 to 18.

The particulate zinc oxide may be formed into a dispersion, in anysuitable aqueous of organic liquid medium. By dispersion is meant a truedispersion, i.e. where the solid particles are stable to aggregation.The particles in the dispersion are relatively uniformly dispersed andresistant to settling out on standing, but if some settling out doesoccur, the particles can be easily redispersed by simple agitation.

Cosmetically acceptable materials are preferred as the liquid medium. Auseful organic medium is a liquid oil such as vegetable oils, e.g. fattyacid glycerides, fatty acid esters and fatty alcohols. A preferredorganic medium is a siloxane fluid, especially a cyclic oligomericdialkylsiloxane, such as the cyclic pentamer of dimethylsiloxane knownas cyclomethicone. Alternative fluids include dimethylsiloxane linearoligomers or polymers having a suitable fluidity andphenyltris(trimethylsiloxy)silane (also known as phenyltrimethicone).

Examples of suitable organic media include avocado oil, C12-15 alkylbenzoate, C12-15 alkyl ethylhexanoate, C12-15 alkyl lactate, C12-15alkyl salicylate, C13-14 isoparaffin, C18-36 acid glycol ester, C18-36acid triglyceride, caprylic/capric glycerides, caprylic/caprictriglyceride, caprylic/capric/lauric triglyceride,caprylic/capric/linoleic triglyceride, caprylic/capric/myristic/stearictriglyceride, caprylic/capric/stearic triglyceride, castor oil, castoroil-silicone ester, cetearyl ethylhexanoate, cetearyl isononanoate,cetearyl palmitate, cetearyl stearate, cetyl dimethicone, cetyldimethicone copolyol, cetyl ethylhexanoate, cetyl glycol isostearate,cetyl isononanoate, cetyl lactate, cetyl myristate, cetyl oleate, cetylpalmitate, cetyl ricinoleate, cetyl stearate, cocoglycerides, coconutoil, cyclomethicone, cyclopentasiloxane, cyclotetrasiloxane, decylisostearate, decyl oleate, decyl polyglucoside, dibutyl adipate,diethylhexyl dimer dilinoleate, diethylhexyl malate, diisopropyladipate, diisopropyl dimer dilinoleate, diisostearyl adipate,diisostearyl dimer dilinoleate, diisostearyl malate, diisostearyltrimethylolpropane siloxy silicate, dilauryl trimethylolpropane siloxysilicate, dimethicone, dimethicone copolyol, dimethicone propylPG-betaine, dimethiconol, dimethyl isosorbide, dioctyl maleate,dioctylodedecyl dimer dilonoleate, ethylhexyl benzoate, ethylhexylcocoate, ethylhexyl dimethyl PABA, ethylhexyl ethylhexanoate, ethylhexylhydroxystearate, ethylhexyl hydroxystearate benzoate, ethylhexylisononanoate, ethylhexyl isopalmitate, ethylhexyl isostearate,ethylhexyl laurate, ethylhexyl methoxycinnamate, ethylhexyl myristate,ethylhexyl neopentanoate, ethylhexyl oleate, ethylhexyl palmitate,ethylhexyl salicylate, ethylhexyl stearate, glyceryl caprate, glycerylcaprylate, glyceryl caprylate/caprate, glyceryl cocoate, glyceryldilaurate, glyceryl dioleate, glyceryl hydroxystearate, glycerylisostearate, glyceryl laurate, glyceryl oleate, glycol oleate, glycolricinoleate, helianthus annuus (hybrid sunflower) seed oil, helianthusannuus (sunflower) seed oil, homosalate, isoamyl laurate, isoamylp-methoxycinnamate, isocetyl alcohol, isocetyl behenate, isocetylethylhexanoate, isocetyl isostearate, isocetyl laurate, isocetyllinoleoyl stearate, isocetyl myristate, isocetyl palmitate, isocetylsalicylate, isocetyl stearate, isocetyl stearoyl stearate,isohexadecane, isononyl isononanoate, isopropyl C12-15-pareth-9carboxylate, isopropyl isostearate, isopropyl lanolate, isopropyllaurate, isopropyl linoleate, isopropyl methoxycinnamate, isopropylmyristate, isopropyl oleate, isopropyl palmitate, isopropylPPG-2-isodeceth-7 carboxylate, isopropyl ricinoleate, isopropylstearate, isostearic acid, isostearyl alcohol, isostearylethylhexanoate, isostearyl isononanoate, isostearyl isostearate,isostearyl lactate, isostearyl myristate, isostearyl neopentanoate,isostearyl palmitate, isostearyl stearoyl stearate, jojoba oil, lanolin(lanolin oil), maleated soybean oil, myristyl isostearate, myristyllactate, myristyl myristate, myristyl neopentanoate, myristyl stearate,octocrylene, octyldecanol, octyidodecanol, oenothera biennis (eveningprimrose oil), paraffinum liquidum (mineral oil), PCA dimethicone,pentaerythrityl tetraisononanoate, pentaerythrityl tetraisostearate,perfluoropolymethylisopropyl ether, persea gratissima (avocado oil),phenyl trimethicone, PPG-15 stearyl ether, propylene glycol ceteth-3acetate, propylene glycol dicaprylate, propylene glycoldicaprylate/dicaprate, propylene glycol dipelargonate, propylene glycoldistearate, propylene glycol isoceteth-3 acetate, propylene glycolisostearate, propylene glycol laurate, proylene glycol ricinoleate,propylene glycol stearate, prunus dulcis (sweet almond oil), squalane,squalene, tricaprylin, tricaprylyl citrate, tridecyl ethylhexanoate,tridecyl neopentanoate, tridecyl stearoyl stearate, triethylhexanoin,triethylhexyl citrate, trihydroxystearin, triisocetyl citrate,triisostearin, triisostearyl citrate, trimethylolpropane triisostearate,trimethylsiloxysilicate, triticum vulgare (wheat germ oil), vitisvinifera (grape) seed oil, and mixtures thereof.

The zinc oxide dispersions may also contain a dispersing agent in orderto improve the properties thereof. The dispersing agent is preferablypresent in the range from 1 to 50%, more preferably 3 to 30%,particularly 5 to 20%, and especially 8 to 15% by weight based on thetotal weight of zinc oxide particles.

Suitable dispersing agents for use in an organic medium includesubstituted carboxylic acids, soap bases and polyhydroxy acids.Typically the dispersing agent can be one having a formula X.CO.AR inwhich A is a divalent bridging group, R is a primary secondary ortertiary amino group or a salt thereof with an acid or a quaternaryammonium salt group and X is the residue of a polyester chain whichtogether with the —CO— group is derived from a hydroxy carboxylic acidof the formula HO—R′—COOH. As examples of typical dispersing agents arethose based on ricinoleic acid, hydroxystearic acid, hydrogenated castoroil fatty acid which contains in addition to 12-hydroxystearic acidsmall amounts of stearic acid and palmitic acid. Dispersing agents basedon one or more polyesters or salts of a hydroxycarboxylic acid and acarboxylic acid free of hydroxy groups can also be used. Compounds ofvarious molecular weights can be used. Other suitable dispersing agentsare those monoesters of fatty acid alkanolamides and carboxylic acidsand their salts. Alkanolamides are based on ethanolamine, propanolamineor aminoethyl ethanolamine for example. Alternative dispersing agentsare those based on polymers or copolymers of acrylic or methacrylicacids, e.g. block copolymers of such monomers. Other dispersing agentsof similar general form are those having epoxy groups in the constituentradicals such as those based on the ethoxylated phosphate esters. Thedispersing agent can be one of those commercially referred to as a hyperdispersant.

Suitable dispersing agents for use in an aqueous medium include apolymeric acrylic acid or a salt thereof. Partially or fully neutralizedsalts are usable e.g. the alkali metal salts and ammonium salts.Examples of dispersing agents are polyacrylic acids, substituted acrylicacid polymers, acrylic copolymers, sodium and/or ammonium salts ofpolyacrylic acids and sodium and/or ammonium salts of acryliccopolymers. Such dispersing agents are typified by polyacrylic aciditself and sodium or ammonium salts thereof as well as copolymers of anacrylic acid with other suitable monomers such as a sulphonic acidderivative such as 2-acrylamido 2-methyl propane sulphonic acid.Comonomers polymerisable with the acrylic or a substituted acrylic acidcan also be one containing a carboxyl grouping. Usually the dispersingagents have a molecular weight of from 1,000 to 10,000 and aresubstantially linear molecules.

One feature of the present invention is that liquid dispersions,particularly in an organic medium, can be produced which contain atleast 30%, suitably at least 40%, preferably at least 50%, morepreferably at least 55%; particularly at least 60%, especially at least65%, and generally up to 75% by weight of the total weight of thedispersion, of zinc oxide particles.

Alternatively, the particulate zinc oxide may be in the form of a lotionor cream of a solid and/or semi-solid dispersion. Suitable solid orsemi-solid dispersions may contain, for example, in the range from 50 to90%, preferably 60 to 85% by weight of particulate zinc oxide accordingto the present invention, together with any one or more of the liquidmedia disclosed herein, or a high molecular polymeric material, such asa wax.

The dispersions of the present invention are useful as ingredients forpreparing sunscreen compositions, especially in the form of emulsions.The dispersion may further contain conventional additives suitable foruse in the intended application, such as conventional cosmeticingredients used in sunscreens.

The particulate zinc oxide described herein may provide the onlyultraviolet light attenuators in a sunscreen product according to theinvention, but other sunscreening agents, such as other metal oxidesand/or other organic materials may also be added. For example, the zincoxide particles described herein may be used in combination withexisting commercially available zinc oxide and/or titanium dioxidesunscreens. Suitable organic sunscreens for use with zinc oxideaccording to the invention include p-methoxy cinnamic acid esters,salicylic acid esters, p-amino benzoic acid esters, non-sulphonatedbenzophenone derivatives, derivatives of dibenzoyl methane and esters of2-cyanoacrylic acid. Specific examples of useful organic sunscreensinclude benzophenone-1, benzophenone-2, benzophenone-3, benzophenone-6,benzophenone-8, benzophenone-12, isopropyl dibenzoyl methane, butylmethoxy dibenzoyl methane, ethyl dihydroxypropyl PABA, glyceryl PABA,ethylhexyl dimethyl PABA, ethylhexyl methoxycinnamate, homosalate,ethylhexyl salicylate, ethylhexyl triazone, octocrylene, etocrylene,menthyl anthranilate, 4-methylbenzylidene camphor, benzophenone 4, andphenyl benzimidazole sulphonic acid.

In a particular preferred embodiment of the present invention, theparticulate zinc oxide described herein is used in combination withtransparent particulate titanium dioxide.

The individual or primary transparent titanium dioxide particles, usedin combination with the zinc oxide particles described herein, arepreferably acicular in shape and have a long axis (maximum dimension orlength) and short axis (minimum dimension or width). The third axis ofthe particles (or depth) is preferably approximately the same dimensionsas the width. The mean length by number of the primary titanium dioxideparticles is suitably in the range from 50 to 90 nm, preferably 55 to 85nm, more preferably 60 to 80 nm, particularly 65 to 77 nm, andespecially 69 to 73 nm. The mean width by number of the particles issuitably in the range from 5 to 20 nm, preferably 8 to 19 nm, morepreferably 10 to 18 nm, particularly 12 to 17 nm, and especially 14 to16 nm. The primary titanium dioxide particles preferably have a meanaspect ratio d₁:d₂ (where d₁ and d₂, respectively, are the length andwidth of the particle) in the range from 2.0 to 8.0:1, more preferably3.0 to 6.5:1, particularly 4.0 to 6.0:1, and especially 4.5 to 5.5:1.

The transparent titanium dioxide suitably has a dispersion particle sizeof (i) median volume particle diameter in the range from 24 to 42 nm,preferably 27 to 39 nm, more preferably 29 to 37 nm, particularly 31 to35 nm, and especially 32 to 34 nm, and/or (ii) less than 16% by volumeof particles having a volume diameter of less than 15 nm, preferablyless than 12 nm, more preferably less than 9 nm, particularly less than6 nm, and especially less than 4 nm below the median volume particlediameter, and/or (iii) less than 30% by volume of particles having avolume diameter of less than 8 nm, preferably less than 6 nm, morepreferably less than 4 nm, particularly less than 3 nm, and especiallyless than 2 nm below the median volume particle diameter, and/or (iv)more than 84% by volume of particles having a volume diameter of lessthan 20 nm, preferably less than 15 nm, more preferably less than 10 nm,particularly less than 7 nm, and especially less than 5 nm above themedian volume particle diameter, and/or (v) more than 70% by volume ofparticles having a volume diameter of less than 8 nm, preferably lessthan 6 nm, more preferably less than 4 nm, particularly less than 3 nm,and especially less than 2 nm above the median volume particle diameter(median volume particle diameter and particle size distribution measuredusing the same technique as for zinc oxide).

In addition, the transparent titanium dioxide particles (i) preferablyhave an extinction coefficient at 524 nm (E₅₂₄) of less than 2.0, morepreferably in the range from 0.1 to 1.0, particularly 0.2 to 0.7, andespecially 0.3 to 0.5 l/g/cm and/or (ii) preferably have an extinctioncoefficient at 450 nm (E₄₅₀) of less than 3.0, more preferably in therange from 0.1 to 2.0, particularly 0.5 to 1.5, and especially 0.7 to1.0 l/g/cm, and/or (iii) suitably have an extinction coefficient at 360nm (E₃₆₀) of greater than 3, preferably in the range from 4 to 10, morepreferably 5 to 8, particularly 5.5 to 7.5, and especially 6 to 7l/g/cm, and/or (iv) preferably have an extinction coefficient at 308 nm(E₃₀₈) of greater than 30, more preferably in the range from 35 to 65,particularly 40 to 60, and especially 45 to 55 l/g/cm, and/or (v)preferably have a maximum extinction coefficient E(max) in the rangefrom 40 to 80, more preferably from 45 to 75, particularly 50 to 70, andespecially 55 to 65 l/g/cm and/or (iv) preferably have a λ(max) in therange from 260 to 290, more preferably 265 to 285, particularly 268 to280, and especially 270 to 275 nm (extinction values measured using thesame basic technique as for zinc oxide).

A particularly preferred transparent titanium dioxide is availablecommercially as Solaveil Clarus (trade mark) from Uniqema.

A preferred embodiment is a dispersion comprising a mixture ofparticulate zinc oxide as defined herein and transparent particulatetitanium dioxide as defined above, suitably at the followingconcentrations; (i) in the range from 1 to 30%, preferably 5 to 25%,more preferably 10 to 20%, particularly 12 to 18%, and especially 14 to16% by weight of zinc oxide, and (ii) in the range from 10 to 50%,preferably 15 to 45%, more preferably 20 to 40%, particularly 25 to 35%,and especially 28 to 32% by weight of titanium dioxide. The ratio byweight of titanium dioxide to zinc oxide in the dispersion is preferablyin the range from 0.5 to 5:1, more preferably 1 to 3:1, particularly 1.5to 2.5:1, and especially 1.8 to 2.2:1.

A preferred sunscreen product comprises a mixture of (i) in the rangefrom 0.1 to 15%, more preferably 0.5 to 10%, particularly 1 to 6%, andespecially 2 to 4% by weight of particulate zinc oxide as definedherein, and (ii) in the range from 0.1 to 15%, more preferably 1 to 10%,particularly 3 to 8%, and especially 5 to 7% by weight of transparentparticulate titanium dioxide as defined above.

Such a sunscreen product comprising a zinc oxide/titanium dioxidemixture can exhibit high UV protection, (i) suitably having a SunProtection Factor (SPF) of greater than 10, preferably in the range from13 to 45, more preferably 17 to 35, particularly 20 to 30, andespecially 23 to 27; and/or reduced whiteness, (i) preferably having achange in whiteness ΔL of less than 4, more preferably less than 3,particularly less than 2.5, and especially less than 2.0.

The invention is illustrated by the following non-limiting examples.

In this specification the following test methods have been used todetermine certain properties of the zinc oxide (and titanium dioxide)particles, and dispersions and sunscreen products containing the zincoxide (and titanium dioxide) particles:

1) Particle Size Measurement of Primary Zinc Oxide Particles

A small amount of zinc oxide, typically 2 mg, was worked intoapproximately 2 drops of an oil, for one or two minutes on a flatsurface using the tip of a steel spatula. The resultant suspension wasdiluted with solvent and a carbon-coated grid suitable for transmissionelectron microscopy was wetted with the suspension and dried on ahot-plate. Approximately 18 cm×21 cm photographs were produced at anappropriate, accurate magnification. Generally about 300-500 particleswere displayed at about 2 diameters spacing. A minimum number of 300primary particles were manually sized using a transparent size gridconsisting of a row of circles of gradually increasing diameter,representing spherical particles. Each circle had ellipses of graduallyincreasing aspect ratio but equal volume beneath it. The outline of eachparticle was then fitted to the appropriate sphere or ellipse and loggedagainst its equivalent spherical diameter. The mean particle diameter,and particle size distribution, of the particles were calculated fromthe above measurements. In addition, the aspect ratio of the particleswas determined from the maximum and minimum dimensions of at least 100particles. Alternatively, the measurements could be performed bycomputerised image analysis.

The basic method assumes log normal distribution standard deviations inthe 1.2-1.6 range (wider crystal size distributions would require manymore crystals to be counted for example of the order of 1000). Thedispersion method described above has been found to be suitable forproducing almost totally dispersed distributions of primary zinc oxideparticles whilst introducing minimal crystal fracture. Any residualaggregates (or secondary particles) are sufficiently well defined thatthey, and any small debris, can be ignored, and effectively only primaryparticles included in the count.

2) Median Particle Volume Diameter and Particle Size Distribution ofZinc Oxide Particles in Dispersion

A dispersion of zinc oxide particles was produced by mixing 9 g ofpolyhydroxystearic acid (weight average molecular weight approximately1750) with 71 g of C12-C15 alkyl benzoate, and then adding 120 g of zincoxide into the solution. The mixture was passed through a horizontalbead mill, operating at approximately 1500 r.p.m. and containingzirconia beads as grinding media for 15 minutes. The dispersion of zincoxide particles was diluted to between 30 and 40 g/l by mixing withisopropyl myristate. The diluted sample was analysed on the BrookhavenBI-XDC particle sizer in centrifugation mode, and the median particlevolume diameter and particle size distribution determined.

3) BET Specific Surface Area of Zinc Oxide Particles

The single point BET specific surface area was measured using aMicromeritics Flowsorb II 2300.

4) Change in Whiteness and Whiteness Index

A sunscreen formulation (as shown in Example 2) was coated on to thesurface of a glossy black card and drawn down using a No 2 K bar to forma film of 12 μm wet thickness. The film was allowed to dry at roomtemperature for 10 minutes and the whiteness of the coating on the blacksurface (L_(F)) measured using a Minolta CR300 colourimeter. The changein whiteness ΔL was calculated by subtracting the whiteness of thesubstrate (L_(S)) from the whiteness of the coating (L_(F)) andexpressing the value relative to the formulation containing 5% by weightof zinc oxide particles. The whiteness index is the percentage change inwhiteness ΔL compared to a standard zinc oxide (=100% value) (Z-Cote (exBASF)).

5) Sun Protection Factor

The Sun Protection Factor (SPF) of a sunscreen formulation wasdetermined using the in vitro method of Diffey and Robson, J. Soc.Cosmet. Chem. Vol. 40, pp 127-133, 1989.

6) Extinction Coefficients

0.02 g of zinc oxide dispersion was made up to 100 ml with cyclohexane.The diluted sample was then placed in a spectrophotometer (Perkin-ElmerLambda 2 UV/VIS Spectrophotometer) with a 1 cm path length and theabsorbance, of UV and visible light measured. Extinction coefficientswere calculated from the equation A=E.c.l, where A=absorbance,E=extinction coefficient in litres per gram per cm, c=concentration ingrams per litre, and l=path length in cm.

EXAMPLES Example 1

Particulate zinc oxide was produced by a modification of the Frenchprocess. A zinc oxide dispersion was prepared by mixing 9 g ofpolyhydroxystearic acid with 71 g C12-C15 alkyl benzoate, and thenadding 120 g of zinc oxide into the solution. The mixture was passedthrough a horizontal bead mill, operating at approximately 1500 r.p.m.and containing zirconia beads as grinding media for 15 minutes. Thedispersion was diluted by adding 200 g of C12-C15 alkyl benzoate andsubjected to a particle fractionation technique to remove largerparticles from the dispersion.

The zinc oxide dispersion had the following extinction coefficients;E₅₂₄ E₃₀₈ E₃₆₀ E(max) λ(max) 1.1 15.4 16.1 16.8 367

Example 2

A typical sunscreen formulation containing zinc oxide can be prepared asfollows; % by weight Phase A: ARLACEL 165 (trade mark, ex Uniqema) 6.0Stearyl alcohol 0.75 SPAN 60 (trade mark, ex Uniqema) 0.75 TWEEN 60(trade mark, ex Uniqema) 1.35 White petroleum jelly 4.0 Silicone200/350cs (ex Dow Corning) 1.0 Light mineral oil 8.0 Propylene glycol5.0 Zinc Oxide dispersion produced in Example 1 15.0 Phase B: Water;Pure 48.9 Keltrol RD (ex Kelco) 0.2 Glycerine BP 5.0 Aloe Vera Gel 10:1(ex A & E Connock) 0.7 Phase C: Water; Pure 2.5 Phenonip (ex Clariant)0.6 Germall 115 (ex ISP Sutton Laboratories) 0.3

Procedure:

-   1. Heat phase A (except for zinc oxide) and phase B to 75° C.-   2. Add zinc oxide to phase A; homogenise.-   3. At 75° C., add A to B with high-shear mixing. Continue mixing for    2 minutes.-   4. Begin cooling with moderate stirring. Add C at 45° C. Cool to    30° C. with stirring.

Example 3

A typical sunscreen formulation containing zinc oxide and transparenttitanium dioxide can be prepared as follows; % by weight Phase A:ARLACEL P135 (trade mark, ex Uniqema) 2.0 ARLAMOL HD (trade mark, exUniqema) 5.8 SOLAVEIL CT-100 (trade mark, ex Uniqema (titanium 11.1dioxide dispersion)) Zinc oxide dispersion produced in Example 1 8.4Candelilla Wax (ex Eggar) 1.0 Phase B: Water; Pure 66.5 MgSO₄.7H₂O 0.7PRICERINE 9091 (trade mark, ex Uniqema) 4.0 Phase C: Germaben II (ex ISPSutton) 0.5

Procedure:

-   1. Heat phases A and B separately to 75-80° C.-   2. Slowly add B to A with intensive stirring.-   3. Homogenise for approximately 2 minutes.-   4. Cool to 45° C. with intensive stirring. Add C. Cool to room    temperature with stirring.

1. A dispersion comprising particles of zinc oxide in a dispersingmedium wherein the zinc oxide particles in dispersion have (i) a medianvolume particle diameter in the range from 70 to 130 nm, (ii) less than16% by volume of particles having a volume diameter of less than 35 nmbelow the median volume particle diameter, and (iii) more than 84% byvolume of particles having a volume diameter of less than 57 nm abovethe median volume particle diameter.
 2. A dispersion according to claim1 wherein the zinc oxide particles have a median volume particlediameter in the range from 80 to 120 nm, preferably 93 to 107 nm.
 3. Adispersion according to claim 1, wherein less than 16% by volume of zincoxide particles have a volume diameter of less than 30 nm, preferably ofless than 25 nm, below the median volume particle diameter.
 4. Adispersion according to claim 1, wherein more than 84% by volume of zincoxide particles have a volume diameter of less than 54 nm, preferably ofless than 50 nm, above the median volume particle diameter.
 5. Adispersion according to claim 1, wherein less than 30% by volume of zincoxide particles have a volume diameter of less than 19 nm, preferably ofless than 16 nm, below the median volume particle diameter.
 6. Adispersion according to claim 1, wherein more than 70% by volume ofparticles have a volume diameter of less than 24 nm, preferably of lessthan 21 nm, above the median volume particle diameter.
 7. A dispersionaccording to claim 1, wherein the zinc oxide particles comprise primaryparticles having a mean particle size in the range from 35 to 65 nm,preferably 45 to 55 nm.
 8. A dispersion according to claim 1, whereinthe zinc oxide particles have a BET specific surface area in the rangefrom 15 to 35, preferably
 20. to 30 m²/g.
 9. A dispersion according toclaim 1, wherein the zinc oxide particles have a lead content of lessthan 10 ppm.
 10. A dispersion according to claim 1, wherein the zincoxide particles have an extinction coefficient at 524 nm (E₅₂₄) of lessthan 1.2, preferably in the range from 0.1 to 1.0.
 11. A dispersionaccording to claim 1, wherein the zinc oxide particles have anextinction coefficient at 450 nm (E₄₅₀) of less than 2.5, preferably inthe range from 0.3 to
 2. 12. A dispersion according to claim 1, whereinthe zinc oxide particles have an extinction coefficient at 360 nm (E₃₆₀)of greater than 10, preferably in the range from 11 to 20 l/g/cm.
 13. Adispersion according to claim 1, wherein the zinc oxide particles havean extinction coefficient at 308 nm (E₃₀₈) of greater than 10,preferably in the range from 11 to 20 l/g/cm.
 14. A dispersion accordingto claim 1, wherein the zinc oxide particles have a maximum extinctioncoefficient E (max) in the range from 10 to 25, preferably 12 to 20l/g/cm.
 15. A dispersion according to claim 1, wherein the zinc oxideparticles have a λ(max) in the range from 360 to 380, preferably 366 to375 nm.
 16. A dispersion according to claim 1, comprising at least 30%by weight of zinc oxide particles.
 17. A particulate zinc oxide having adispersion particle size of (i) median volume particle diameter in therange from 70 to 130 nm, (ii) less than 16% by volume of particleshaving a volume diameter of less than 35 nm below the median volumeparticle diameter, and (iii) more than 84% by volume of particles havinga volume diameter of less than 57 nm above the median volume particlediameter.
 18. A particulate zinc oxide having an extinction coefficientat 524 nm (E₅₂₄) in the range from 0.1 to 1.0 l/g/cm, an extinctioncoefficient at 450 nm (E₄₅₀) in the range from 0.3 to 2 l/g/cm, anextinction coefficient at 360 nm (E₃₆₀) in the range from 11 to 20l/g/cm, an extinction coefficient at 308 nm (E₃₀₈) in the range from 11to 20 l/g/cm, a maximum extinction coefficient E(max) in the range from12 to 20 l/g/cm, and a λ(max) in the range from 363 to 377 nm.
 19. Asunscreen product comprising a zinc oxide or dispersion as defined inclaim
 1. 20. A sunscreen product according to claim 19 which istransparent when applied to the skin and has a change in whiteness AL inthe range from 0.3 to 3, preferably 0.9 to 2.0.
 21. A sunscreen productaccording to claim 19 having a whiteness index in the range from 5 to80%, more preferably 15 to 60%.
 22. The use of a zinc oxide ordispersion as defined in claim 1 in the manufacture of a sunscreenhaving reduced whiteness.
 23. A sunscreen product comprising (a) zincoxide having a dispersion particle size of (i) median volume particlediameter in the range from 70 to 130 nm, (ii) less than 16% by volume ofparticles having a volume diameter of less than 35 nm below the medianvolume particle diameter, and (iii) more than 84% by volume of particleshaving a volume diameter of less than 57 nm above the median volumeparticle diameter, and (b) titanium dioxide having a dispersion particlesize of (i) median volume particle diameter in the range from 24 to 42nm, (ii) less than 16% by volume of particles having a volume diameterof less than 15 nm below the median volume particle diameter, and (iii)more than 84% by volume of particles having a volume diameter of lessthan 20 nm above the median volume particle diameter.
 24. A sunscreenproduct according to claim 23 wherein the titanium dioxide particles indispersion (i) have less than 30% by volume of particles having a volumediameter of less than 8 nm, preferably less than 6 nm, below the medianvolume particle diameter, and/or (ii) have more than 70% by volume ofparticles having a volume diameter of less than 8 nm, preferably lessthan 6 nm, above the median volume particle diameter
 25. A sunscreenproduct according to claim 23, wherein the titanium dioxide particles(i) have an extinction coefficient at 524 nm (E₅₂₄) of less than 2.0,preferably in the range from 0.1 to 1.0 l/g/cm, and/or (ii) have anextinction coefficient at 450 nm (E₄₅₀) of less than 3.0, preferably inthe range from 0.1 to 2.0 l/g/cm, and/or (iii) have an extinctioncoefficient at 360 nm (E₃₆₀) of greater than 3, preferably in the rangefrom 4 to 10 l/g/cm, and/or (iv) have an extinction coefficient at 308nm (E₃₀₈) of greater than 30, preferably in the range from 35 to 65l/g/cm, and/or (v) have a maximum extinction coefficient E(max) in therange from 40 to 80, preferably from 45 to 75 l/g/cm, and/or (iv) have aλ(max) in the range from 260 to 290, preferably 265 to 285 nm.
 26. Asunscreen product according to claim 23, comprising in the range from 3to 8% by weight of particulate titanium dioxide and in the range from 1to 6% by weight of particulate zinc oxide.